By Francois Murphy VIENNA (Reuters) -One of the Russian-held Zaporizhzhia nuclear power plant’s four main power lines has been repaired and is once again supplying the plant with electricity from the Ukrainian grid two weeks after it went down, the U.N. nuclear watchdog said on Saturday. Even though the six reactors at Zaporizhzhia, Europe’s biggest nuclear power plant, have been shut down, the fuel in them still needs cooling to avoid a potentially catastrophic meltdown. That means the plant needs electricity to pump water through the core of the reactors. The power supply at Zaporizhzhia has…